Automatically switching to a different, redundant system upon failure or abnormal termination of the currently active system. Failover can be applied to a cluster of servers, to network or storage components or any other set of redundant devices that must provide high availability because down-time would be expensive or inconvenient. It may be implemented in hardware, software or a combination.
A "hot standby" is continuously active at the same time as the failed system, using some kind of load balancing to share the work, whereas a "warm standby" is ready to become active at short notice.
When the failed system is operational again it may "failback", i.e. become (one of) the active system(s) or it may become a warm standby.